Pretty straightforward, allow the use of in game gold and doubloons to be exchanged for ancient coins for the emporium.
Make the exchange rate something like:
1 million gold in exchange for 100 ancient coins
1000 doubloons in exchange for 100 ancient coins.
There's already the ancient skeletons who drop ancient coins. Why not make gold and doubloons more useful.
I know in my circle of friends gold and doubloons are nearly worthless due to the over saturation of those currencies. The bilge rat black market does help as a currency sink, but why not do more. Give more incentive to collect anything and everything and I feel the game could be revitalized.